1. Field of the Invention
This invention relates to the protection of electrical circuitry from reverse engineering, and more particularly to the protection of access codes embedded within integrated circuitry.
2. Description of the Related Art
It is very important to prevent unauthorized access to certain portions of electrical circuits, particularly integrated circuits. For example, digital codes or other data may be stored in certain portions of the circuit to prevent its unauthorized use; the circuit will function properly only if the user enters the appropriate code. Such circuitry may be compromised, however, by various methods of analysis, such as visual inspection, microprobing, secondary electron emission voltage-contrast analysis, etc. A number of techniques have been used in the past to prevent such reverse engineering.
Perhaps the most basic technique is to enclose the circuit in plastic encapsulation or protective die coating. However, access to the circuitry can be gained by mechanical or acid drilling through the encapsulation material. Another approach is to add misleading circuit topology so as to disguise the portion of the circuitry to be protected, or to customize each different die with its own specific code. An example of this approach is disclosed in U.S. Pat. No. 4,766,516, "Method and Apparatus for Securing Integrated Circuits from Unauthorized Copying and Use," Aug. 23, 1988, to Ozdemir et al. and assigned to Hughes Aircraft Company. These techniques may retard, but generally do not totally prevent, successful reverse engineering. U.S. Pat. No. 4,766,516 also discloses a use control scheme for a secure system which has circuitry on several printed circuit boards. Each circuit transmits a control code to enable the circuitry on the next board in order when it itself has received its control code; an external software verification unit transmits the control code for the first board. Overall system operation is disabled if any of the boards are tampered with or missing.
Other methods include changing the circuit chip at regular intervals, and encapsulating the chip with wires that easily break if an attempt is made to open the encapsulation and probe the chip. Such methods are discussed in W. Diffie and M. E. Hellman, "Privacy and Authentication: An Introduction to Cryptography", Proc. IEEE, Vol. 67, No. 3, March, 1979.